Hi,
I travel on ticketless flights all the time, and have never been asked for my credit card at checkin.
The only issue was - once I was trying to use the Automated Self Checkin, and in order to print my ticket, the machine requested the credit card which has been used to book the flight. My card had actually been stolen and I had cancelled it, so it meant I couldnt do self check-in and had to go to the counter to checkin.
Your son should be ok, but ring the airline just to be sure.